Why does it need 1 layer instead of N? When there is a fault, each layer only cares about its own tokens, all the remote are worthless.

Yea but fundamentally how would you construct the chain? Let’s say it’s a rollup that wanted to do this for Ethereum and Solana Each bridge needs to have a notion of what the STF for the rollup is, if each one goes off it’s own DA layer, they wouldn’t be the same rollup